The invention relates to gas generating compositions, especially for use in safety devices for vehicles. In particular, the invention relates to such compositions based on guanidine nitrate used in pedestrian protection devices.
Gas generating compositions based on guanidine nitrate are known, for example, from EP-A 1 006 096 and U.S. Pat. No. 6,143,102, Almost all manufacturers of vehicle occupant restraint systems make use of such compositions in series production of pyrotechnical inflators for airbag modules of occupant protection devices, especially for driver and passenger airbags. The compositions comprising guanidine nitrate as fuel commonly used for said applications have a mass-related gas yield of approx. 65 to 75%, bum rates of approx. 20 to 30 mm/s and combustion temperatures of about 1700 to 2000 K. The share of guanidine nitrate in these compositions frequently is within the range of from 40 to 50 % by weight.
For the use of airbag modules in the interior of the vehicle increased requirements to the generated propellant are applicable, because the latter can enter into the passenger compartment via discharge orifices in the airbag. The limits of gas components such as CO, NH3 and NOx required m the specifications of the car manufacturers can only be reached by fuel mixtures having a substantially balanced oxygen balance. Therefore the percentage of guanidine nitrate in the gas generating compositions for airbag modules is limited to approx. 65% by weight, and apart from guanidine nitrate considerable percentages of oxidizing agents have to be employed. Since the oxidizing agents usually do not completely contribute to the gas generation, the possible gas yield of those compositions is reduced. Also for compositions having low burn temperatures which positively influence the noxious gas composition a higher percentage of non-gas generating additives has to be accepted, which equally affects the gas yield. The gas yields that can be obtained by the gas generating compositions comprising guanidine nitrate as fuel common so far thus do not amount to more than 75 %.
U.S. Pat. No. 6,893,517 describes gas generating compositions based on guanidine nitrate having a gas yield of about 80 %. These compositions contain further organic fuels and are formulated for use in belt tensioners. The compositions therefore have a definitely higher burn temperature of more than 2000 K and a high burn rate of more than 40 mm/s at 20 MPa.
Pure guanidine nitrate shows no self-retaining and complete disintegration after ignition. Therefore U.S. Pat. No. 2,604,391 suggests improving the ignition behavior and the burn characteristics of guanidine nitrate by appropriate additives. By the addition of copper and copper compounds to guanidine nitrate, gas generating compositions having a burn rate of about 1mm/s at 0.9 MPa are obtained. By adding vanadium pentaoxide the burn rate is intended to be increased. However, vanadium pentaoxide is toxic and is excluded from applications in the automotive industry.
The burn rates required for gas generating compositions for occupant protection systems (driver, passenger and side impact systems) in general are within >20 mm/s. As a result, in the commonly used fabric thicknesses of the propellant members (in general tablets) of 1 to 2 mm the burning is completed after a maximum of 100 ms. In this case, by the fabric thickness the layer thickness of a propellant member is understood which is passed during burning of the propellant member until complete consumption thereof. For example, a cylindrical tablet having a diameter of 6 mm and a height of 3 mm has a fabric thickness of 1,5 mm, as in the case of even burning from all sides after burning of a layer thickness of 1.5 mm the entire tablet is consumed.
However, for pedestrian protection applications at the exterior of the vehicle to protect the pedestrian against impacting on the hood or the windscreen, gas delivery times of more than 100 ms are desired. Moreover the inflated airbag is intended to have an as long durability as possible. For this purpose, it is advantageous when the temperature of the gas generated for inflating the airbag is as low as possible so as to keep the pressure loss due to cooling of the gas low.
It is the object of the invention to provide gas generating compositions by which longer gas delivery times and an improved durability of the protective devices activated by the release of gas can be achieved and which are suited for pedestrian protection applications.
This object is achieved by a gas generating composition according to claim 1.
The subject matter of the invention also is a pedestrian protection device in vehicles in which the gas generating composition according to the invention is contained as well as the use of the gas generating compositions according to the invention in pedestrian protection devices of vehicles.
Advantageous embodiments of the invention which can be optionally combined with each other are stated in the subclaims.